Hydrogen Gas Fueled Vehicles A Step Closer

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While liquid hydrogen is denser and takes up less space, it is very expensive and difficult to produce. It also reduces the environmental benefits of hydrogen vehicles. Widespread commercial acceptance of these vehicles will require finding the right material that can store hydrogen gas at high volumetric and gravimetric densities in reasonably sized light-weight fuel tanks.

While liquid hydrogen is denser and takes up less space, it is very
expensive and difficult to produce. It also reduces the environmental
benefits of hydrogen vehicles. Widespread commercial acceptance of
these vehicles will require finding the right material that can store
hydrogen gas at high volumetric and gravimetric densities in reasonably
sized light-weight fuel tanks.

Researchers at the UCLA Henry Samueli School of Engineering and
Applied Science, with the use of molecular dynamics simulations, have
solved a decade old mystery that could one day lead to commercially
practical designs of storage materials for use in hydrogen gas fueled
vehicles.

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In 1997, it was discovered that adding a small amount of titanium to
a well-known metal hydride, sodium alanate, not only lowers the
temperature of hydrogen release from the material but also allows for
an easy refueling and storage of high density hydrogen at reasonable
pressures and temperatures. In fact, the weight percent of stored
hydrogen was instantly doubled in comparison with other inexpensive
materials.

"Nobody really understood what the titanium did. The chemical
processes and the mechanisms were really a mystery," said Vidvuds
Ozolins, associate professor of material science and engineering, a
member of the California NanoSystems Institute, and lead author of the
study.*

With computers and the power of basic physics, chemistry and quantum
mechanics, Ozolins' group decided to take a step back and analyze the
sodium alanate in its pure form, without added titanium. The group
analyzed the atomic processes occurring in the material and what
happens to the chemical bond between the hydrogen and the material at
the temperatures of hydrogen release. The computation gave the
researchers information that would have been very difficult to obtain
experimentally.

The computation suggested a reaction mechanism that is essential for
the extraction of hydrogen from the material which involves diffusion
of aluminum ions within the bulk of the hydride. By comparing the
calculated activation energies to the experimentally determined values,
Ozolins' group found that aluminum diffusion is the key rate limiting
process in materials catalyzed with titanium. Thus, titanium
facilitates processes in the material that are essential for turning on
this mechanism and extracting hydrogen at lower temperatures.

"This method and this knowledge can now be used to analyze other
materials that would make for better storage systems than sodium
alanate. We are still on the fundamental end of the study. But if we
can figure this out computationally, the people with the technology in
engineering can figure out the rest," said Hakan Gunaydin, a UCLA
graduate student in Ozolins' lab and another one of the study's authors.

"Sodium alanate in itself is a prototypical complex hydride with a
reasonable storage density and very good kinetics. Hydrogen goes in and
comes out quickly but it wouldn't be practical for a car simply because
it doesn't contain enough hydrogen. So that's why we are so interested
in understanding how the hydrogen comes out, what happens exactly and
how we can take this to other materials," said Ozolins.

What Ozolins' group, along with UCLA chemistry and biochemistry
professor Kendall Houk, also a member of the California NanoSystems
Institute, hopes to do now is to apply the methods and lessons learned
to those materials that would make for a commercially practical
hydrogen gas storage system. They hope their findings will one day
facilitate the design and creation of an affordable and environmentally
friendly hydrogen vehicle.

*The study appears on the Proceedings of the National Academy of Sciences web site on February 27.

The study was funded by the U.S. Department of Energy and the National Science Foundation.